Production of a GaN bulk crystal substrate and a semiconductor device formed on a GaN bulk crystal substrate

ABSTRACT

A method of making a bulk crystal substrate of a GaN single crystal includes the steps of forming a molten flux of an alkali metal in a reaction vessel and causing a growth of a GaN single crystal from the molten flux, wherein the growth is continued while replenishing a compound containing N from a source outside the reaction vessel.

BACKGROUND OF THE INVENTION

[0001] The present invention generally relates to semiconductor devicesand more particularly to a semiconductor device having a GaN bulkcrystal substrate.

[0002] GaN is a III-V compound semiconductor material having a largebandgap of blue to ultraviolet wavelength energy. Thus, intensiveinvestigations are being made with regard to development of opticalsemiconductor devices having a GaN active layer for use particularly inoptical information storage devices including a digital video datarecorder (DVD). By using such a light emitting semiconductor deviceproducing blue to ultraviolet wavelength optical radiation for theoptical source, it is possible to increase the recording density ofoptical information storage devices.

[0003] Conventionally, a laser diode or light-emitting diode having aGaN active layer has been constructed on a sapphire substrate in view ofthe absence of technology of forming a GaN bulk crystal substrate.

[0004]FIG. 1 shows the construction of a conventional GaN laser diodeaccording to Nakamura, S., et al., Jpn. J. Appl. Phys. vol.36 (1997)pp.L1568-L1571, Part 2, No.12A, Dec. 1, 1997, constructed on a sapphiresubstrate 1.

[0005] Referring to FIG. 1, the sapphire substrate 1 has a (0001)principal surface covered by a low-temperature GaN buffer layer 2, andincludes a GaN buffer layer 3 of n-type grown further on the bufferlayer 2. The GaN buffer layer 3 includes a lower layer part 3 a and anupper layer part 3 b both of n-type, with an intervening SiO₂ maskpattern 4 provided such that the SiO₂ mask pattern 4 is embedded betweenthe lower layer part 3 a and the upper layer part 3 b. Morespecifically, the SiO₂ mask pattern 4 is formed on the lower GaN bufferlayer part 3 a, followed by a patterning process thereof to form anopening 4A in the SiO₂ mask pattern 4.

[0006] After the formation of the SiO₂ mask pattern 4, the upper GaNlayer part 3 b is formed by an epitaxial lateral overgrowth (ELO)process in which the layer 3 b is grown laterally on the SiO₂ mask 4.Thereby, desired epitaxy is achieved with regard to the lower GaN layerpart 3 a at the opening 4A in the SiO₂ mask pattern 4. By growing theGaN layer part 3 b as such, it is possible to prevent the defects, whichare formed in the GaN layer part 3 a due to the large lattice misfitbetween GaN and sapphire, from penetrating into the upper GaN layer part3 b.

[0007] On the upper GaN layer 3 b, a strained super-lattice structure 5having an n-type Al_(0.14)Ga_(0.86)N/GaN modulation doped structure isformed, with an intervening InGaN layer 5A of the n-type having acomposition In_(0.1)Ga_(0.9)N interposed between the upper GaN layerpart 3 b and the strained superlattice structure 5. By providing thestrained superlattice structure 5 as such, dislocations that areoriginated at the surface of the sapphire substrate 1 and extending inthe upward direction are intercepted and trapped.

[0008] On the strained superlattice structure 5, a lower cladding layer6 of n-type GaN is formed, and an active layer 7 having an MQW structureof In_(0.01)Ga_(0.98)N/In_(0.15)Ga_(0.85)N is formed on the claddinglayer 6. Further, an upper cladding layer 8 of p-type GaN is formed onthe active layer 7, with an intervening electron blocking layer 7A ofp-type AlGaN having a composition of Al_(0.2)Ga_(0.8)N interposedbetween the active layer 7 and the upper cladding layer 8.

[0009] On the upper cladding layer 8, another strained superlatticestructure 9 of a p-type Al_(0.14)Ga_(0.86)N/GaN modulation dopedstructure is formed such that the superlattice structure 9 is covered bya p-type GaN cap layer 10. Further, a p-type electrode 11 is formed incontact with the cap layer 10 and an n-type electrode 12 is formed incontact with the n-type GaN buffer layer 3 b.

[0010] It is reported that the laser diode of FIG. 1 oscillatessuccessfully with a practical lifetime, indicating that the defectdensity in the active layer 7 is reduced successfully.

[0011] On the other hand, the laser diode of FIG. 1 cannot eliminate thedefects completely, and there remain substantial defects particularly incorrespondence to the part on the SiO₂ mask 4 as represented in FIG. 2.See Nakamura S. et al., op cit. It should be noted that such defectsformed on the SiO₂ mask 4 easily penetrate through the strainedsuperlattice structure 5 and the lower cladding layer 6 and reach theactive layer 7.

[0012] In view of the foregoing concentration of the defects in thecentral part of the SiO₂ mask pattern 4, the laser diode of FIG. 1 usesthe part of the semiconductor epitaxial structure located on the opening4A of the SiO₂ mask 4, by forming a mesa structure M in correspondenceto the opening 4A. However, the defect-free region formed on the opening4A has a lateral size of only several microns, and thus, it is difficultto construct a high-power laser diode based on the construction ofFIG. 1. When the laser diode of FIG. 1 is driven at a high power, thearea of optical emission in the active region extends inevitably acrossthe defects, and the laser diode is damaged as a result of opticalabsorption caused by the defects. Further, the laser diode of FIG. 1having such a construction has other various drawbacks associated withthe defects in the semiconductor epitaxial layers, such as largethreshold current, limited lifetime, and the like. Further, the laserdiode of FIG. 1 has a drawback, in view of the fact that the sapphiresubstrate is an insulating substrate, in that it is not possible toprovide an electrode on the substrate. As represented in FIG. 1, it isnecessary to expose the top surface of the n-type GaN buffer layer 3 byan etching process in order to provide the n-type electrode 12, whilesuch an etching process complicates the fabrication process of the laserdiode. In addition, the increased distance between the active layer 7and the n-type electrode 12 causes the problem of increased resistanceof the current path, while such an increased resistance of the currentpath deteriorates the high-speed response of the laser diode.

[0013] Further, the conventional laser diode of FIG. 1 suffers from theproblem of poor quality of mirror surfaces defining the optical cavity.Due to the fact that the sapphire single crystal constituting thesubstrate 1 belongs to hexagonal crystal system, formation of theoptical cavity cannot be achieved by a simple cleaving process. It hasbeen therefore necessary to form the mirror surfaces, when fabricatingthe laser diode of FIG. 1 by conducting a dry etching process, while themirror surface thus formed by a dry etching process has a poor quality.

[0014] Because of the foregoing reasons, as well as because of othervarious reasons not mentioned here, it is desired to form the substrateof the GaN laser diode by a bulk crystal GaN and form the laser diodedirectly on the GaN bulk crystal substrate.

[0015] With regard to the art of growing a bulk crystal GaN, there is asuccessful attempt reported by Porowski (Porowski, S., J. Crystal Growth189/190 (1998) pp.153-158, in which a GaN bulk crystal is synthesizedfrom a Ga melt under an elevated temperature of 1400-1700° C. and anelevated N₂ pressure of 12-20 kbar (1.2-2 GPa). This process, however,can only provide an extremely small crystal in the order of 1 cm indiameter at best. Further the process of Porowski requires a speciallybuilt pressure-resistant apparatus and a long time is needed for loadingor unloading a source material, or increasing or decreasing the pressureand temperature. Thus, the process of this prior art would not be arealistic solution for mass-production of a GaN bulk crystal substrate.It should be noted that the reaction vessel of Porowski has to withstandthe foregoing extremely high pressure, which is rarely encountered inindustrial process, under the temperature exceeding 1400° C.

[0016] Further, there is a known process of growing a GaN bulk crystalwithout using an extremely high pressure environment for growing a GaNbulk crystal as reported by Yamane, H., et al., Chem. Mater. 1997, 9,413-416. More specifically, the process of Yamane et al. successfullyavoids the use of the extremely high-pressure used in Porowski, byconducting the growth of the GaN bulk crystal from a Ga melt in thepresence of a Na flux.

[0017] According to the process of Yamane, a metallic Ga source and aNaN₃ (sodium azide) flux are confined in a pressure-resistance reactionvessel of stainless steel together with a N₂ atmosphere, and thereaction vessel is heated to a temperature of 600-800° C. and held for aduration of 24-100 hours. As a result of the heating, the pressureinside the reaction vessel is elevated to the order of 100 kg/cm² (about10 MPa), which is substantially lower than the pressure used byPorowski. As a result of the reaction, GaN crystals are precipitatedfrom the melt of a Na—Ga system. In view of the relatively low pressureand low temperature needed for the reaction, the process of Yamane etal. is much easier to implement.

[0018] On the other hand, the process of Yamane relies upon theinitially confined N₂ molecules in the atmosphere and the N atomscontained in the NaN₃ flux for the source of N. Thus, when the reactionproceeds, the N₂ molecules in the atmosphere or the N atoms in the Na—Gamelt are depleted with the precipitation of the GaN crystal, and thereappears a limitation in growing a large bulk crystal of GaN. The GaNcrystals obtained by the process of Yamane et al. typically have a sizeof 1 mm or less in diameter. Thus, the process of Yamane et al. op cit.,while being successful in forming a GaN bulk crystal at a relatively lowpressure and temperature, cannot be used for a mass production of a GaNsubstrate in the industrial base.

SUMMARY OF THE INVENTION

[0019] Accordingly, it is a general object of the present invention toprovide a novel and useful GaN semiconductor device having a bulkcrystal substrate wherein the foregoing problems are eliminated.

[0020] Another and more specific object of the present invention is toprovide a process of making a bulk crystal substrate of a GaN singlecrystal.

[0021] Another object of the present invention is to provide a processof fabricating a GaN semiconductor device having a bulk crystalsubstrate of a GaN single crystal.

[0022] Another object of the present invention is to provide a bulkcrystal substrate of a single crystal GaN.

[0023] Another object of the present invention is to provide an opticalsemiconductor device having a bulk crystal substrate of a GaN singlecrystal.

[0024] Another object of the present invention is to provide an electrondevice having a bulk crystal substrate of a GaN single crystal.

[0025] Another object of the present invention is to provide anapparatus for making a bulk crystal substrate of a GaN single crystal.

[0026] According to the present invention, a high-quality GaN bulkcrystal substrate is obtained with a process suitable formass-production, by continuously supplying N so as to compensate for thedepletion of N occurring in the system in which precipitation of a GaNsingle crystal takes place. By using the GaN bulk crystal substrate thusobtained, it is possible to fabricate an optical semiconductor devicethat produces an optical radiation of blue to ultraviolet wavelengthwith a large optical power. Further, the GaN bulk crystal substrate canbe used as a substrate of an electron device such as HEMT.

[0027] Other objects and further features of the present invention willbecome apparent from the following detailed description when read inconjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 is a diagram showing the construction of a conventionallaser diode constructed on a sapphire substrate;

[0029]FIG. 2 is a diagram showing the problem associated with the laserdiode of FIG. 1;

[0030]FIG. 3 is a diagram showing the construction of a growth apparatusused in a first embodiment of the present invention for growing a GaNbulk crystal;

[0031]FIGS. 4A and 4B are diagrams showing a part of the apparatus ofFIG. 3 in detail;

[0032]FIG. 5 is a diagram showing a cathode luminescent spectrum of aGaN bulk crystal obtained in the first embodiment;

[0033]FIG. 6 is a diagram showing a control of GaN composition in thegrowth apparatus of FIG. 3;

[0034]FIG. 7 is a diagram showing the construction of a growth apparatusused in a second embodiment of the present invention for growing a GaNbulk crystal;

[0035]FIG. 8 is a diagram showing the construction of a growth apparatusused in a third embodiment of the present invention for growing a GaNbulk crystal;

[0036]FIG. 9 is a diagram showing the construction of a growth apparatusused in a fourth embodiment of the present invention for growing a GaNbulk crystal;

[0037]FIG. 10 is a diagram showing the construction of a growthapparatus used in a fifth embodiment of the present invention forgrowing a GaN bulk crystal;

[0038]FIG. 11 is a diagram showing the construction of a growthapparatus used in a sixth embodiment of the present invention forgrowing a GaN bulk crystal;

[0039]FIG. 12 is a diagram showing the construction of a growthapparatus used in a seventh embodiment of the present invention forgrowing a GaN bulk crystal;

[0040]FIG. 13 is a diagram showing the construction of a seed crystalused in the growth apparatus of FIG. 12;

[0041]FIG. 14 is a diagram showing the construction of a growthapparatus used in an eighth embodiment of the present invention forgrowing a GaN bulk crystal;

[0042]FIGS. 15A and 15B are diagrams showing a part of the growthapparatus of FIG. 14;

[0043]FIG. 16 is a diagram showing the growth apparatus of FIG. 14 inthe state in which a growth of the GaN bulk crystal has been made;

[0044]FIG. 17 is a diagram showing the construction of a growthapparatus used in a ninth embodiment of the present invention forgrowing a GaN bulk crystal;

[0045]FIG. 18 is a diagram showing the construction of a growthapparatus used in a tenth embodiment of the present invention forgrowing a GaN bulk crystal;

[0046]FIG. 19 is a diagram showing X-ray diffraction data obtained for aGaN bulk crystal according to an eleventh embodiment of the presentinvention;

[0047]FIG. 20 is a diagram showing the construction of a laser diodehaving a GaN bulk crystal substrate according to a twelfth embodiment ofthe present invention; and

[0048]FIG. 21 is a diagram showing the construction of a HEMT having aGaN bulk crystal substrate according to a thirteenth embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

[0049] [FIRST EMBODIMENT]

[0050]FIG. 3 shows the construction of a growth apparatus 100 used in afirst embodiment of the present invention for growing a GaN bulkcrystal.

[0051] Referring to FIG. 3, the growth apparatus 100 includes apressure-resistant reaction vessel 101 typically of a stainless steelhaving an inner diameter of about 75 mm and a length of about 300 mm andaccommodates therein a crucible 102 of Nb or BN. As will be explainedlater, the crucible 102 is loaded with a starting material of metallicGa and a NaN₃ flux and is confined in the reaction vessel 101 togetherwith an N₂ atmosphere 107. Further, the reaction vessel 101 is suppliedwith N₂ or a gaseous compound of N from an external source via aregulator valve 109 and an inlet 108. The reaction vessel 101 thusloaded with the starting material in the crucible 102 is heated byenergizing heaters 110 and 111 to a temperature of 650-850° C., and thepressure inside the reaction vessel is regulated to a moderate value ofabout 5 MPa by controlling the valve 109. By holding the temperature andthe pressure, a precipitation of GaN bulk crystal takes place from aNa—Ga melt, which is formed in the crucible 102 as a result of themelting of the starting material.

[0052]FIG. 4A shows the loading of the starting material in the crucible102, while FIG. 4B shows the state in which the source material hascaused a melting.

[0053] Referring to FIG. 4A, a high-purity metallic Ga and a high-puritymetallic Na are weighed carefully and loaded into the crucible 102,wherein the foregoing process of weighing and loading are conducted inthe N₂ atmosphere. It is also possible to use high-purity NaN₃ in placeof high-purity metallic Na source.

[0054] In the state of FIG. 4B, on the other hand, there appears a melt102A of the Na—Ga system in the crucible 102 and crystallization of GaNtakes place from various parts of the melt 102A including a free surfaceof the melt and a sidewall or bottom wall of the crucible 102. There, itwas observed that a large single crystal 102B of GaN grows on the meltfree surface contacting with the atmosphere and fine needle-like GaNcrystals 102C grow on the sidewall or bottom wall of the crucible 102.

[0055] With the growth of the GaN crystals, particularly with the growthof the GaN single crystal 102B, N in the atmosphere is consumed and thepressure inside the reaction vessel gradually falls as a result ofdepletion of N in the atmosphere. Thus, in the present embodiment, thedepletion of N in the atmosphere 107 is compensated for by replenishingN₂ or a compound of N such as NH₃ from an external source. Thereby, thegrowth of the GaN single crystal 102B continues at the melt free surfaceand a large GaN single crystal suitable for use in an opticalsemiconductor device such as a laser diode or light-emitting diode as aGaN bulk crystal substrate is obtained. The construction of FIG. 3 caneasily produce the GaN single crystal 102B with a thickness of 100 μm ormore. The GaN single crystal 102B thus formed at the temperature of650-850° C. has a hexagonal crystal symmetry.

[0056]FIG. 5 shows the cathode luminescent spectrum of the GaN singlecrystal 102B thus obtained in comparison with the cathode luminescentspectrum of a GaN thick film grown on a sapphire substrate or an SiCsubstrate.

[0057] Referring to FIG. 5, it can be seen that the GaN crystal 102B ofthe present embodiment shows a distinct and strong peak corresponding tothe band edge of GaN at the wavelength of about 360 nm. Further, it canbe seen that no other peak exists in the GaN single crystal 102B of thepresent embodiment. The result of FIG. 5 indicates that the GaN crystal102B thus formed has a defect density of less than 10²-10³ cm⁻³. Thus,the GaN single crystal 102B is suitable for use as a bulk GaN substrateof various optical semiconductor devices including a laser diode and alight-emitting diode as noted already. Hereinafter, the GaN singlecrystal 102B will be called a GaN bulk crystal in view of application toa GaN bulk crystal substrate.

[0058] Contrary to the present embodiment, the GaN thick film formed onthe sapphire substrate or formed on the SiC substrate shows a remarkablepeak at the wavelength of about 600 nm corresponding to deep impuritylevels. This clearly indicates that the GaN thick film thus formed on asapphire substrate or an SiC substrate contains a substantial amount ofdefects. Associated with the high level of defects, it can be seen thatthe peak strength for the band edge is substantially smaller than thecase of the GaN bulk crystal 102B of the present embodiment.

[0059] In the growth process of FIG. 4B, it should be noted that thereappears also an intermetallic compound 102D of GaNa along the sidewalland bottom surface of the crucible 102 indicated in FIG. 4B by a brokenline. Thus, the region represented in FIG. 4B by the broken line in factincludes the fine GaN crystals 102C and the GaNa intermetallic compound102D in the form of a mixture. The GaN fine crystals 102C or the GaNaintermetallic compound 102D thus formed releases Ga into the melt 102A,and the Ga atoms thus released contribute to the growth of the GaN bulkcrystal 102B when transported to the melt surface.

[0060] Thus, by continuously replenishing N₂ or NH₃, the growth processof the GaN bulk crystal 102B continues until Ga in the melt 102A is usedup.

[0061]FIG. 6 shows the control of the N₂ pressure in the atmosphere 107with the growth of the GaN bulk crystal 102B from the melt 102A.

[0062] Referring to FIG. 6, it can be seen that the N₂ pressure anecessary for maintaining the stoichiometric composition for the GaNbulk crystal 102B changes depending on the Ga content in the melt 102Arepresented in the horizontal axis. When the N₂ pressure in theatmosphere 107 is fixed (a₁=a₂), it is not possible to maintain thestoichiometric composition for the GaN bulk crystal 102B. Thus, thepresent invention changes the N₂ pressure a in the atmosphere 107 withthe progress of growth of the GaN bulk crystal 102B as represented asa₁≠a₂.

[0063] [Second Embodiment]

[0064]FIG. 7 shows the construction of a growth apparatus 200 accordingto a second embodiment of the present invention, wherein those partscorresponding to the parts described previously are designated by thesame reference numerals and the description there of will be omitted.

[0065] Referring to FIG. 7, the present embodiment uses heaters 111A and111B in place of the heater 111 and induces a temperature gradient inthe melt 102A for facilitating transport of Ga from the GaN finecrystals 102C or the GaNa intermetallic compound 102D to the meltsurface.

[0066] More specifically, the heater 111B is provided in correspondenceto the bottom part of the crucible 102 and controls, together with theheart 11A, the melt temperature at the bottom part of the crucible 102lower than the melt surface. As a result of energization of the heaters111A and 111B, a temperature gradient shown in FIG. 7 is induced.

[0067] Due to the increased temperature at the bottom part of thecrucible 102, undesirable precipitation of GaN crystals on bottomsurface of the crucible 102 is minimized, and the growth of the GaN bulkcrystal 102B on the melt surface is promoted substantially. When a GaNfine crystal 102C is formed, such a GaN fine crystal 102C is immediatelydissolved into the melt 102A and no substantial deposition occurs on thebottom part of the crucible 102. Further, the intermetallic compound ofGaNa, formed at a temperature lower than about 530° C., acts also as thesource of Ga and Na in the melt 102A.

[0068] Similarly to the first embodiment, the GaN bulk crystal 102Bformed according to the present embodiment has a defect density in theorder of 102-10³ cm⁻³ or less. Thus, the GaN bulk crystal 102B issuitable for a bulk GaN substrate of various optical semiconductordevices including a laser diode and a light-emitting diode.

[0069] [Third Embodiment]

[0070]FIG. 8 shows the construction of a growth apparatus 300 accordingto a third embodiment of the present invention, wherein those partscorresponding to the parts described previously are designated by thesame reference numerals and the description thereof will be omitted.

[0071] Referring to FIG. 8, the present embodiment is a modification ofthe embodiment of FIG. 7 and uses the heaters 110 and 111, describedwith reference to the growth apparatus 100 for inducing the desiredtemperature gradient. As other aspects of the present embodiment aresubstantially the same as those of the previous embodiment, furtherdescription will be omitted.

[0072] [Fourth Embodiment]

[0073]FIG. 9 shows the construction of a growth apparatus 400 accordingto a fourth embodiment of the present invention, wherein those partscorresponding to the parts described previously are designated by thesame reference numerals and the description thereof will be omitted.

[0074] Referring to FIG. 9, the growth apparatus 400 has a constructionsimilar to that of FIG. 3, except that there is provided a container 103holding a metallic Ga source 104 inside the reaction vessel 101. Thecontainer 103 is provided at a first end of a tube 103A extendingoutside of the reaction vessel 101, and there is provided a pressureregulator 106 at a second, opposite end of the tube 103. The pressureregulator 106 is supplied with a pressurized N₂ gas from an externalsource and causes a molten Ga, formed in the container 103 as a resultof heating, to drip to the Na—Ga melt 102A in the crucible 102 via ahole 105 formed at a bottom part of the container 103.

[0075] According to the construction of FIG. 9, depletion of Ga in themelt 102A is replenished from the Ga source 104 and a thickness of 300μm or more is obtained for the GaN bulk crystal 102B as a result of thecontinuous crystal growth.

[0076] Similarly to the previous embodiments, the GaN bulk crystal 102Bformed according to the present embodiment has a defect density of10²-10³ cm⁻³ or less. Thus, the GaN bulk crystal 102B of the presentembodiment is suitable for use as a bulk GaN substrate of variousoptical semiconductor devices including a laser diode and alight-emitting diode.

[0077] [Fifth Embodiment]

[0078]FIG. 10 shows the construction of a growth apparatus 500 accordingto a fifth embodiment of the present invention, wherein those partscorresponding to the parts described previously are designated by thesame reference numerals and the description thereof will be omitted.

[0079] Referring to FIG. 10, the growth apparatus 500 has a constructionsimilar to that of the growth apparatus of FIG. 9, except that there isprovided an outer pressure vessel 112 outside the reaction vessel 101,and the space between the reaction vessel 101 and the outer pressurevessel 112 is filled with a pressurized gas such as N₂, which isintroduced via a regulator 114 and an inlet 113.

[0080] By providing the pressure vessel 112 outside the reaction vessel101, the pressurized reaction vessel 101 is supported from outside andthe design of the reaction vessel 101 becomes substantially easier. Asrepresented in FIG. 10, there is provided a thermal insulator 115between the heater 110 or 111 and the outer pressure vessel 112 and thetemperature rise of the pressure vessel 112 is avoided. Thereby, thepressure vessel 112 maintains a large mechanical strength even when theinner, reaction vessel 101 is heated to the temperature exceeding 600 or700° C. In order to avoid the decrease of mechanical strength, it ispossible to provide a water cooling system (not shown) on the outerpressure vessel 112.

[0081] The outer pressure vessel 112 can be provided also to the growingapparatuses 100-300 explained before as well as to the growingapparatuses to be described hereinafter.

[0082] As other features of the present embodiment are substantially thesame as those of the previous embodiments, further description thereofwill be omitted.

[0083] [Sixth Embodiment]

[0084]FIG. 11 shows the construction of a growing apparatus 600according to a sixth embodiment of the present invention, wherein thoseparts corresponding to the parts described previously are designated bythe same reference numerals and the description thereof will be omitted.

[0085] Referring to FIG. 11, the growing apparatus 600 has aconstruction similar to that of the growing apparatus 100 of FIG. 3,except that there is provided a holder 601 holding a Ga—Na melt outsidethe reaction vessel 101 and the Ga—Na melt in the holder 601 is suppliedinto the reaction vessel 101 and to the melt 102A in the crucible 102via a tube 601A penetrating through a wall of the reaction vessel 101,in response to a pressurization of the holder 601 by a pressurized gassuch as an N₂ gas supplied via a line 602.

[0086] According to the present embodiment, the depletion of Ga in themelt 102A is replenished together with the Na flux, and the growth ofthe GaN bulk crystal 102B at the free surface of the melt 102A isconducted continuously. It should be noted that depletion of N in thesystem is also replenished by the external N source similarly to theprevious embodiments. As a result, a high-quality GaN bulk crystalsuitable for use as a substrate of various optical semiconductor devicesis obtained with a thickness well exceeding 100 μm, generally about 300μm or more.

[0087] [Seventh Embodiment]

[0088]FIG. 12 shows the construction of a growth apparatus 700 accordingto a seventh embodiment of the present invention, wherein those partscorresponding to the parts described previously are designated by thesame reference numerals and the description thereof will be omitted.

[0089] Referring to FIG. 12, the growing apparatus 700 has aconstruction similar to that of the growing apparatus 600 of theprevious embodiment, except that there is provided a rod 702 carrying aseed crystal 701 at a tip end thereof in contact with the free surfaceof the melt 102A in the crucible 102. Further there is provided a motor703 for pulling up the rod 702, and there occurs a continuous growth ofthe GaN bulk crystal 102B at the melt surface with the pulling up of therod 702. Thereby, an ingot of a GaN bulk crystal is obtained.

[0090] By slicing the GaN bulk crystal ingot thus obtained, it ispossible to mass produce the GaN bulk crystal substrate for use invarious optical semiconductor devices including a laser diode and alight-emitting diode.

[0091]FIG. 13 shows an example of the seed crystal 701 provided at thetip end of the rod 702.

[0092] Referring to FIG. 13, the seed crystal 702 is formed to have aslab shape with a width w and a thickness d corresponding to the widthand thickness of the GaN substrate to be formed. Thus, by pulling up therod 702 straight in the upward direction, a slab-shaped GaN bulk crystalis grown continuously. Thus, by merely polishing the surface of the GaNbulk crystal slab, followed by a cleaving process, it is possible tomass-produce the GaN bulk crystal substrate having a quality suitablefor use in various optical semiconductor devices including a laser diodeand a light-emitting diode.

[0093] As other features of the present embodiment is more or less thesame as those of the previous embodiments, further description thereofwill be omitted.

[0094] [Eighth Embodiment]

[0095]FIG. 14 shows the construction of a growing apparatus 800according to an eighth embodiment of the present invention, whereinthose parts corresponding to the parts described previously aredesignated by the same reference numerals and the description thereofwill be omitted.

[0096] Referring to FIG. 14, the growing apparatus 800 has aconstruction similar to that of the growing apparatus 700 of theprevious embodiment, except that a cover member 803 is provided so as tocover the free surface of the melt 102A. Further, the container 601 of aNa—Ga melt is eliminated and a container 801 having a heating mechanism801A and containing therein a molten Na is provided outside the reactionvessel 101. Thereby, a vapor of Na is supplied from the container 801into the interior of the reaction vessel 101 via a tube 802 and the Navapor is added to the atmosphere 107 therein.

[0097] According to the present embodiment, uncontrolled precipitationof the GaN fine crystals 102C on the sidewall or bottom surface of thecrucible (see FIG. 4B) is minimized, by controlling the vapor pressureof Na from the container 801. Further, no GaN precipitation occurs onthe melt free surface, as the free surface of the melt 102A is coveredby the cover member 803, except for a central part of the melt wherethere is formed an opening 803A in the cover member 803 for allowing theseed crystal 701 on the rod 702 to make a contact with the surface ofthe melt 102A.

[0098] Thus, according to the construction of FIG. 14, the Na vapor fluxacts selectively at the part of the melt 102A where the growth of thebulk GaN ingot is made, and the uncontrolled precipitation of the GaNfine crystals 102C is effectively suppressed.

[0099] It should be noted that cover member 803 has a variable geometryconstruction formed of a number of small, fan-shaped members, in whichthe opening 803A can be changed with the growth of the GaN bulk crystal102B in the form of ingot by moving the fan-shaped members in adirection of an arrow Q as represented in FIGS. 15A and 15B, whereinFIG. 15A shows the state in which the central opening 803A of the covermember 803 is closed while FIG. 15B shows the state in which the opening803A has been expanded for allowing the growth of the GaN bulk crystalingot 102B as represented in FIG. 16. It should be noted that FIG. 16shows the growing apparatus 800 in the state that there occurred agrowth of the GaN bulk crystal 102B in the form of ingot.

[0100] [Ninth Embodiment]

[0101]FIG. 17 shows the construction of a growing apparatus 900according to a tenth embodiment of the present invention, wherein thoseparts corresponding to the parts described previously are designated bythe same reference numeral and the description thereof will be omitted.

[0102] Referring to FIG. 17, the growing apparatus 900 has aconstruction similar to that of the growing apparatus 800 of theprevious embodiment, except that the tube 802 supplying the Na vaporflux has a sleeve part 802A surrounding the rod 702. The sleeve part802A extends along the rod 702 and has an opening 802C in correspondenceto the surface of the melt 102A where the opening 803A is formed in thecover member 803 for the growth of the GaN bulk crystal 102B.

[0103] According to the construction of FIG. 17, the Na flux is suppliedselectively to the part where the growth of the GaN bulk crystal 102Btakes place and an efficient growth becomes possible.

[0104] As other aspects of the present embodiment are the same as thoseof the previous embodiment, further description thereof will be omitted.

[0105] [Tenth Embodiment]

[0106]FIG. 18 shows the construction of a growing apparatus 1000according to a ninth embodiment of the present invention wherein thoseparts corresponding to the parts described previously are designated bythe same reference numerals and the description thereof will be omitted.

[0107] Referring to FIG. 18, the growing apparatus 1000 has aconstruction similar to that of the growing apparatus 700 of FIG. 12,except that the rod 702 driven by the motor 703 and pulling up the seed701 in the upward direction is replaced by a rod 702′ driven by a motor703′ and pulls down a seed 701′ in the downward direction. Thus, asrepresented in FIG. 18, the GaN bulk crystal 102B forms an ingot growninside the melt 102A. As other aspects of the present invention is thesame as those described before, further description of the presentembodiment will be omitted.

[0108] [Eleventh Embodiment]

[0109] In any of the foregoing first through tenth embodiments, thegrown of the GaN bulk crystal 102B has been achieved at the temperatureof 650-850° C. under the presence of a Na flux. As mentioned before, theGaN bulk crystal 102B thus obtained has a symmetry of hexagonal crystalsystem.

[0110] On the other hand, the inventor of the present invention hasdiscovered that a cubic GaN crystal is obtained as the bulk GaN crystal102B provided that the growth is made at a temperature of less than 600°C. under the presence of Na, or when the growth is made at a temperatureof 650-850° C. under the presence of K. K may be introduced into thesystem in the form of a high-purity metallic K starting material,similarly to the case represented in FIG. 4A.

[0111]FIG. 19 shows X-ray diffraction data obtained for a GaN bulkcrystal grown by the apparatus of FIG. 3 as the bulk crystal 102B at atemperature of 750° C. under the total pressure of 7 MPa in the reactionvessel 101. In FIG. 19, it should be noted that the Fo represents thestructural factor obtained from the diffraction pattern for each of thereflections (h k l), while Fc represents the structural factorcalculated from a cubic zinc blende structure. From the diffractionpattern, it was confirmed that the cubic GaN bulk crystal 102B thusformed has a lattice constant a₀ of 4.5062±0.0009A. Thus, in thecalculation of the foregoing structural factor Fc, the lattice constanta₀ of 4.5062±0.0009A is assumed as the basis of the calculation.Further, FIG. 19 shows an error factor s defined ass = ∑Fo − Fc/∑F  b.

[0112] Referring to FIG. 19, it can be seen that there is an excellentagreement between the observed structural factor and the calculatedstructural factor assuming the cubic zinc blende structure for theobtained GaN bulk crystal 102B. It can be safely concluded that the GaNbulk crystal 102B obtained in the present embodiment is a 100% cubic GaNcrystal. From the X-ray diffraction analysis, existence of hexagonal GaNcrystal was not detected. Further it was confirmed that the cubic GaNbulk crystal 102B thus formed provides a cathode luminescent peaksubstantially identical with the spectrum of FIG. 5. In other words, thecubic GaN bulk crystal of the present embodiment contains little deepimpurity levels or defects and has an excellent quality characterized bya defect density of 10²-10³ cm⁻³ or less.

[0113] In view of increasing defect density in the GaN crystals grown atlow temperatures, and further in view of the fact that a mixture ofcubic GaN and hexagonal GaN appears when the growth of the GaN bulkcrystal is conducted at the temperature of 600° C. or lower underpresence of Na flux, it is preferred to grow a cubic GaN bulk crystal atthe temperature of 650-850° C. under presence of a K flux.

[0114] [Twelfth Embodiment]

[0115]FIG. 20 shows the construction of a laser diode 150 ofedge-emission type according to a twelfth embodiment of the presentinvention.

[0116] Referring to FIG. 20, the laser diode 150 is constructed on a GaNbulk crystal substrate 151 produced in any of the process explainedbefore. More specifically, the GaN bulk crystal substrate 151 has a highcrystal quality characterized by a defect density of 10²-10³ cm⁻³ orless.

[0117] On the GaN bulk crystal substrate 151, there is provided a lowercladding layer 152 of n-type AlGaN epitaxially with respect to thesubstrate 151 and an optical waveguide layer 153 of n-type GaN is formedon the lower cladding layer 152 epitaxially.

[0118] On the optical waveguide layer 153, there is provided an activelayer 154 of MQW structure including an alternate stacking of quantumwell layers of undoped InGaN having a composition represented asIn_(x)Ga_(1-x)N (x=0.15) and barrier layers of undoped InGaN having acomposition represented as In_(y)Ga_(1-y)N (y=0.02). The active layer154 is covered by an optical waveguide layer 155 of p-type GaN, and anupper cladding layer 156 of p-type AlGaN is formed epitaxially on theoptical waveguide layer 155. Further, a contact layer 157 of p-type GaNis formed on the upper cladding layer 156.

[0119] The contact layer 157 and the underlying upper cladding layer 156are subjected to a patterning process to form a loss-guide structureextending in the axial direction of the laser diode 150 and theloss-guide structure thus formed is covered by an SiO₂ film 158. TheSiO2 film 158 is formed with an opening 158A extending in the laseraxial direction for exposing the contact layer 157, and a p-typeelectrode 159 is provided on the SiO₂ film 158 in contact with thecontact layer 157 at the opening 158A.

[0120] Further, an n-type electrode 160 is provided at a bottom surfaceof the GaN bulk crystal substrate 151.

[0121] After forming the laser structure as such, the layeredsemiconductor body including the GaN substrate 151 and the epitaxiallayers 151-157 is subjected to a cleaving process to form mirrorsurfaces M1 and M2 defining an optical cavity. Thereby, the laser diodeproduces a blue to ultraviolet optical beam as a result of stimulatedemission and optical amplification occurring in the optical cavity, asrepresented in FIG. 20 by an arrow.

[0122] According to the present invention, the optical cavity is formedby a simple cleaving process and the quality of the mirror surfaces M1and M2 defining the optical cavity is improved substantially. Thereby,threshold of laser oscillation is lowered substantially. Further, thelaser diode 150 carries the n-type electrode on the bottom surface ofthe GaN bulk crystal substrate 151 and the process of fabricating thelaser diode is improved substantially. As the epitaxial layers,particularly the GaN optical waveguide layers 153 and 155 and the activelayer 154 sandwiched between the layer 153 and 155 are formedepitaxially on the GaN bulk crystal substrate containing only a verysmall amount of defects, the quality of the crystal constituting theforegoing layers 153-155 is improved substantially over the conventionallaser diode of FIG. 1 and the laser diode 150 of FIG. 20 can be drivenwith a large power. Further, the laser diode 150 of the presentembodiment has an improved lifetime over the conventional laser diode ofFIG. 1.

[0123] It should be noted that the GaN bulk crystal substrate 151 may beany of the hexagonal type or cubic type. In view of the easiness ofcleaving process, on the other hand, it is preferable to form the GaNbulk crystal substrate 151 according to the process of the eleventhembodiment by using a K flux.

[0124] Based on the structure of FIG. 20, it is also possible toconstruct a light-emitting diode. Further, it is possible to construct avertical cavity laser diode, which produces a laser beam in a directionvertical to the epitaxial layers, also by using the GaN bulk crystalsubstrate of the present invention.

[0125] In the case of a vertical cavity laser diode, a pair of mirrorsurfaces defining an optical cavity are formed by the epitaxial layerson the GaN bulk crystal substrate 151, and an optical window is formedin the electrode 159. In such a case, the GaN substrate 151 may have athickness larger than 100 μm such as 300 μm or more.

[0126] In the laser diode of FIG. 20, it is also possible to form themirror surfaces M1 and M2 by a dry etching process.

[0127] [Thirteenth Embodiment]

[0128]FIG. 21 shows the construction of an electron device 170constructed on a GaN bulk crystal substrate 171 according to athirteenth embodiment of the present invention.

[0129] Referring to FIG. 21, the electron device 170 is an FET, and theGaN bulk crystal 102B of any of the foregoing first through twelfthembodiments is used for the GaN substrate 171.

[0130] On the substrate 171, there is provided a high-resistanceepitaxial layer 172 of AlN, and a buffer layer 173 of undoped GaN isformed epitaxially on the AlN high-resistance layer 172.

[0131] On the buffer layer 173, a lower barrier layer 174 of undopedAlGaN is formed epitaxially, and a channel layer 175 of undoped GaN isformed on the lower barrier layer 174 such that the channel layer 175 issandwiched between the lower barrier layer 174 and an upper barrierlayer 176 of undoped AlGaN formed epitaxially on the channel layer 175.

[0132] The upper barrier layer 176 is covered by a contact layer 177 ofn-type GaN wherein the layers 174-177 are patterned to form a mesaregion for device isolation. Further, the contact layer 177 is patternedto expose the upper barrier layer 176 in correspondence to the channelregion, and a Schottky electrode 178 of a Ni/Au structure is provided incontact with the exposed upper barrier layer 176 as the gate electrode.Further, ohmic electrodes 179 and 180 of a Ti/Al structure are formed onthe contact layer 177 at both lateral sides of the gate electrode 178 asa source electrode and a drain electrode, respectively.

[0133] In operation, a two-dimensional electron gas is induced in thechannel layer 175 in response to application of a gate voltage to thegate electrode 178. In this state, the FET is turned on.

[0134] According to the present invention, it is thus possible toconstruct an active device such as an FET on a GaN substrate, by usingthe GaN bulk crystal for the substrate. As the GaN bulk crystal producedaccording to the present invention has an high crystal qualitycharacterized by a defect density of 10²-10³ cm⁻³ or less, the problemof severe leakage current that would occur when an FET is constructed ona conventional GaN epitaxial layer formed on a sapphire substrate or anSiC substrate, is successfully eliminated. Further, the construction ofFIG. 21 is advantageous in view of the fact that the electron density ofthe two-dimensional electron gas induced in the channel layer 175 isincreased due to enhanced piezoelectric effect and associated increaseof degree of electron confinement into the channel layer. When thechannel layer contains a high concentration of defects, there occurs alattice relaxation and the effect of carrier confinement is degradedinevitably.

[0135] Further, the GaN bulk crystal of the present invention can beused also as the GaN substrate of other various electron devicesincluding a HEMT, MESFET and an HBT. In fact, the structure of FIG. 21can be modified to form a HEMT by employing an n-type AlGaN layer forthe upper barrier layer 176.

[0136] Further, the present invention is by no means limited to theembodiments described heretofore, but various variations andmodifications may be made without departing from the scope of theinvention.

What is claimed is:
 1. A method of producing a single crystal body of agroup III nitride, comprising the steps of: forming a molten flux of avolatile metal element in a reaction vessel confining therein saidmolten flux together with an atmosphere containing N (nitrogen), suchthat said molten flux contains a group III element in addition to saidvolatile metal element; growing a nitride of said group III element inthe form of a single crystal body in said molten flux; and supplying acompound containing N into said reaction vessel from a source locatedoutside said reaction vessel.
 2. A method as claimed in claim 1, whereinsaid compound comprises N₂ and NH₃.
 3. A method as claimed in claim 1,wherein said volatile metal is an alkali metal.
 4. A method as claimedin claim 1, wherein said volatile metal element is Na.
 5. A method asclaimed in claim 1, wherein said volatile metal element is K.
 6. Amethod as claimed in claim 1, wherein said molten flux further containstherein a source of said group III element at a location away from amelt surface of said molten flux, said step of growing said nitridesingle crystal body including the steps of decomposing said source so asto cause said source to release said group III element into said moltenflux, and transporting said group III element from said source to saidmelt surface through said molten flux.
 7. A method as claimed in claim 6wherein said step of transporting said group III element includes a stepof inducing a temperature gradient in said molten flux such that saidmolten flux has a temperature lower than a temperature of said meltsurface in a part of said molten flux in which said solid source islocated.
 8. A method as claimed in claim 6, wherein said solid source isan intermetallic compound of said group III element and said volatilemetal element.
 9. A method as claimed in claim 6 wherein said solidsource is a nitride of said group III element.
 10. A method as claimedin claim 1, wherein said step of supplying said compound containing Ninto said reaction vessel is conducted such that said single crystalbody grown in said molten flux maintains a predetermined stoichiometry.11. A method as claimed in claim 1, further comprising the step ofsupplying said group III element into said molten flux from a sourcelocated outside said molten flux.
 12. A method as claimed in claim 11,wherein said step of supplying said group III element into said moltenflux is conducted by supplying a melt of said group III element intosaid molten flux.
 13. A method as claimed in claim 11, wherein said stepof supplying said group III element into said molten flux is conductedby supplying a melt of said group III element and said volatile metalelement into said molten flux.
 14. A method as claimed in claim 1,wherein said step of growing said single crystal body of nitridecomprises the steps of contacting a seed crystal with said molten fluxand pulling up said seed crystal from said molten flux in an upwarddirection with a progress of growth of said single crystal body on saidseed crystal.
 15. A method as claimed in claim 1, wherein said step ofgrowing said single crystal body comprises the steps of contacting aseed crystal with said molten flux and pulling down said seed crystalinto said molten flux in a downward direction with a progress of growthof said single crystal body on said seed crystal.
 16. A method asclaimed in claim 1, further comprising a step of supplying a vapor ofsaid volatile metal element into said reaction vessel from an externalsource.
 17. A method of producing a single crystal body of a cubic GaN,comprising the steps of: forming a molten flux of K in a reaction vesselconfining therein said molten flux together with an atmospherecontaining N (nitrogen), such that said molten flux contains Ga inaddition to K; and precipitating a single crystal body of cubic GaN insaid molten flux.
 18. A method as claimed in claim 17, furthercomprising the step of supplying a compound containing N (nitrogen) intosaid reaction vessel from an external source outside said reactionvessel.
 19. A method as claimed in claim 17, wherein said precipitationis conducted by controlling a temperature of a melt surface of saidmolten flux at 650-850° C.
 20. A method of fabricating a semiconductordevice having a bulk crystal substrate of a nitride comprising the stepof: forming a molten flux of a volatile metal element in a reactionvessel confining therein said molten flux together with an atmospherecontaining N (nitrogen), such that said molten flux contains a group IIIelement in addition to said volatile metal element; growing a nitridebulk crystal of said group III element in said molten flux; andsupplying a compound containing N into said reaction vessel from asource located outside said reaction vessel.
 21. A method as claimed inclaim 20, wherein said compound comprises N₂ and NH₃.
 22. A method asclaimed in claim 20, wherein said volatile metal is an alkali metal. 23.A method as claimed in claim 20, wherein said volatile metal element isNa.
 24. A method as claimed in claim 20, wherein said volatile metalelement is K.
 25. A method as claimed in claim 20, wherein said moltenflux further contains therein a source of said group III element at alocation away from a melt surface of said molten flux, said step ofgrowing said nitride bulk crystal including the steps of decomposingsaid source so as to cause said source to release said group III elementinto said molten flux, and transporting said group III element from saidsource to said melt surface through said molten flux.
 26. A method asclaimed in claim 25 wherein said step of transporting said group IIIelement includes a step of inducing a temperature gradient in saidmolten flux such that said molten flux has a temperature lower than atemperature of said melt surface in a part of said molten flux in whichsaid source is located.
 27. A method as claimed in claim 25, whereinsaid solid source is an intermetallic compound of said group III elementand said volatile metal element.
 28. A method as claimed in claim 25wherein said solid source is a nitride of said group III element.
 29. Amethod as claimed in claim 20, wherein said step of supplying saidcompound containing N into said reaction vessel is conducted such thatsaid nitride bulk crystal of said group III element growing in saidmolten flux maintains a predetermined stoichiometry.
 30. A method asclaimed in claim 20, further comprising the step of supplying said groupIII element into said molten flux.
 31. A method as claimed in claim 20,wherein said step of supplying said group III element into said moltenflux is conducted by supplying a melt of said group III element intosaid molten flux from a source located outside said molten flux.
 32. Amethod as claimed in claim 20, wherein said step of supplying said groupIII element into said molten flux is conducted by supplying a melt ofsaid group III element and said volatile metal element into said moltenflux from a source located outside said molten flux.
 33. A method asclaimed in claim 20, wherein said step of precipitating said bulkcrystal comprises the steps of contacting a seed crystal with saidmolten flux and pulling up said seed crystal from said molten flux in anupward direction with a progress of growth of said bulk crystal on saidseed crystal.
 34. A method as claimed in claim 20, wherein said step ofprecipitating said bulk crystal comprises the steps of contacting a seedcrystal with said molten flux and pulling down said seed crystal intosaid molten flux in a downward direction with a progress of growth ofsaid bulk crystal on said seed crystal.
 35. A method as claimed in claim20, further comprising a step of supplying a vapor of said volatilemetal element into said reaction vessel from a source located outsidesaid reaction vessel.
 36. A method of fabricating a semiconductor devicehaving a bulk crystal substrate of a cubic GaN, comprising the step of:forming a molten flux of K in a reaction vessel confining therein saidmolten flux together with an atmosphere containing N (nitrogen), suchthat said molten flux contains Ga in addition to K; and growing a bulkcrystal of GaN of a cubic crystal system at a melt surface of saidmolten flux.
 37. A method as claimed in claim 36, further comprising thestep of supplying a compound containing N (nitrogen) into said reactionvessel from a source located outside said reaction vessel.
 38. A methodas claimed in claim 36, wherein said precipitation is conducted bycontrolling a temperature of said melt surface at 650-850° C.
 39. A bulkcrystal substrate of GaN, comprising: a slab of a GaN single crystalhaving a substantially uniform composition of GaN in a thicknessdirection of said slab, said slab having a defect density lower thanabout 10⁻³ cm⁻³.
 40. A bulk crystal substrate of GaN as claimed in claim39, wherein said slab has a thickness exceeding about 100 μm
 41. A bulkcrystal substrate of GaN as claimed in claim 39 wherein said slab has athickness exceeding about 300 μm.
 42. A bulk crystal substrate of GaN asclaimed in claim 39, wherein said slab has a defect density lower thanabout 10⁻² cm⁻³.
 43. A bulk crystal substrate of GaN as claimed in claim39, wherein said slab is formed of GaN of a hexagonal crystal system.44. A bulk crystal substrate of GaN as claimed in claim 39, wherein saidslab is formed of GaN of a cubic crystal system.
 45. An opticalsemiconductor device, comprising: a bulk crystal substrate of a GaNsingle crystal; and an active layer formed over said bulk crystalsubstrate with epitaxy to said bulk crystal substrate.
 46. An opticalsemiconductor device as claimed in claim 45, wherein said single crystalof GaN constituting said bulk crystal substrate belongs to a hexagonalcrystal system.
 47. An optical semiconductor device as claimed in claim45, wherein said GaN single crystal constituting said bulk crystalsubstrate belongs to a cubic crystal system.
 48. An opticalsemiconductor device as claimed in claim 45, wherein said bulk crystalsubstrate of GaN has a defect density smaller than about 10³ cm⁻³. 49.An optical semiconductor device as claimed in claim 45, wherein saidbulk crystal substrate of GaN has a defect density smaller than about10² cm⁻³.
 50. An optical semiconductor device as claimed in claim 45,wherein said bulk crystal substrate has a thickness exceeding about 100μm.
 51. An optical semiconductor device as claimed in claim 45, whereinsaid bulk crystal substrate has a thickness exceeding about 300 μm. 52.A laser diode, comprising: a bulk crystal substrate of a GaN singlecrystal having a first conductivity type; a lower cladding layer of saidfirst conductivity type formed epitaxially on said bulk crystalsubstrate; an active layer formed epitaxially on said lower claddinglayer; an upper cladding layer of a second conductivity type formedepitaxially on said active layer; a first electrode contacting saidupper cladding layer; a second electrode provided on a bottom surface ofsaid bulk crystal substrate of GaN; and a pair of mirror surfaces facingeach other.
 53. A laser diode as claimed in claim 52, wherein said pairof mirror surfaces are cleaved surfaces.
 54. An laser diode as claimedin claim 52, wherein said GaN single crystal constituting said bulkcrystal substrate belongs to a hexagonal GaN crystal system.
 55. A laserdiode as claimed in claim 52, wherein said GaN single crystalconstituting said bulk crystal substrate belongs to a cubic crystalsystem.
 56. A laser diode as claimed in claim 52, wherein said bulkcrystal substrate of GaN has a defect density smaller than about 10³cm⁻³.
 57. A laser diode as claimed in claim 52, wherein said bulkcrystal substrate of GaN has a defect density smaller than about 10²cm⁻³.
 58. A laser diode as claimed in claim 52, wherein said bulkcrystal substrate has a thickness exceeding about 100 μm.
 59. A laserdiode as claimed in claim 52, wherein said bulk crystal substrate has athickness exceeding about 300 μm.
 60. A laser diode as claimed in claim52, wherein said active layer has a multiple quantum well structure. 61.A laser diode as claimed in claim 60, wherein said active layer furtherincludes a pair of optical waveguide layers below and above saidmultiple quantum well structure.
 62. An electron device, comprising: abulk crystal substrate of a GaN single crystal; an epitaxial layer of anitride formed on said bulk crystal substrate; and an active part formedin said epitaxial layer for switching a flow of carriers transportedthrough said epitaxial layer.
 63. An electron device as claimed in claim62, wherein said epitaxial layer includes a channel layer of GaN formedepitaxially with respect to said bulk crystal substrate, and whereinsaid active part includes a gate electrode provided over said channellayer in correspondence to a channel region defined therein, a sourceelectrode provided over said channel layer at a first side of said gateelectrode, said source electrode injecting carriers into said channellayer, and a drain electrode provided over said channel layer at asecond side of said gate electrode, said drain electrode collectingcarriers from said channel layer.
 64. An electron device as claimed inclaim 63, wherein said epitaxial layer further includes a barrier layerof a nitride formed epitaxially on said channel layer, and wherein saidgate electrode is provided in Schottky contact with said barrier layer.65. An apparatus for growing a group III nitride bulk crystal,comprising: a reaction vessel having a space therein for holding acrucible; a supply line connected to said reaction vessel, said supplyline supplying a pressurized gas of a compound containing N (nitrogen)into said reaction vessel; and a heater disposed outside said reactionvessel, said heater heating said reaction vessel externally so as toform a molten flux of a volatile metal element and a group III elementin said crucible.
 66. An apparatus as claimed in claim 65, furthercomprising a pressure-resistant vessel enclosing said reaction vessel.67. An apparatus as claimed in claim 65, further including a mechanismfor supplying a melt of said group III element into said molten flux insaid crucible.
 68. An apparatus as claimed in claim 67, wherein saidmechanism including a container disposed in said space of said reactionvessel at a location above a surface of said molten flux, said containerhaving an opening for allowing said melt of said group III element tofall into said molten flux.
 69. An apparatus as claimed in claim 67,wherein said mechanism further supplies a melt of said volatile metalelement together with said melt of said group III element.
 70. Anapparatus as claimed in claim 65, further comprising a mechanism forsupplying a vapor of said volatile metal element into said reactionvessel.
 71. An apparatus as claimed in claim 65, further comprising arod adapted for carrying a seed crystal at a tip end and a motor formoving said rod in an upward direction, said rod and said motor beinglocated above a melt surface of said molten flux formed in saidcrucible.
 72. An apparatus as claimed in claim 71, wherein said motor islocated outside said reaction vessel.
 73. An apparatus as claimed inclaim 71, further comprising a cover member covering a surface of saidmolten formed in said crucible, said cover member having a centralopening for allowing said seed crystal to make a contact with saidmolten flux.
 74. An apparatus as claimed in claim 73 wherein said covermember has a variable geometry for changing a size of said centralopening.
 75. An apparatus as claimed in claim 65, further comprising arod adapted for carrying a seed crystal at a tip end, said rod beinginserted into said crucible through a bottom part of said crucible, anda motor provided outside of said reaction vessel for moving said rod ina downward direction.
 76. An apparatus as claimed in claim 65, whereinsaid heater induces a temperature gradient in said molten flux in saidcrucible such that a temperature of said molten flux at a bottom part ofsaid crucible is higher than a temperature at a top surface of saidmolten flux.
 77. An apparatus as claimed in claim 76, wherein saidheater includes a first heater part heating a sidewall of said reactionvessel and a second heater part heating a bottom part of said reactionvessel.